python3Packages.cryptg: fix build on darwin

pkgs/development/python-modules/cryptg/default.nix

@@ -1,9 +1,11 @@
 { lib
+, stdenv
 , buildPythonPackage
 , fetchFromGitHub
 , pythonOlder
 , rustPlatform
 , setuptools-rust
+, libiconv
 }:
 
 buildPythonPackage rec {
@@ -32,6 +34,10 @@ buildPythonPackage rec {
     rust.cargo
   ];
 
+  buildInputs = lib.optionals stdenv.isDarwin [
+    libiconv
+  ];
+
   # has no tests
   doCheck = false;